HOW TO KEEP A COMET FROM HITTING THE EARTH

By Ed Perley

The Problem

In recent years there has been considerable discussion about
the possibility of a comet hitting our world and causing widespread destruction.
Some have discussed ways in which a comet on a collision course with the earth could be
destroyed or diverted. The most obvious suggestion has been to use missiles with
nuclear warheads to shatter the comet or change it’s course. Another suggestion
was to deposit giant rockets on the comet’s surface and fire them to divert it. Both
of these ideas have problems. First, just breaking up the comet would not be much
benefit, unless the resulting fragments were very small. The Earth would get hit by
a shotgun instead of a bullet. Using giant rockets might work, but the logistics of
placing rockets with sufficient thrust to divert a comet would be very difficult and
expensive.

Actually there is a serious problem with trying to blow up a comet with
nuclear explosives. Thermonuclear explosions in space do not work the same as
explosions in the Earth’s atmosphere. A nuclear explosion results from the
sudden disintegration of uranium 235 to form cesium 140 and rubidium 33. The
nuclear reaction produces an extremely dense flux of gamma and Xrays. Because
the atmosphere is opaque to this radiation, a relatively small volume of atmosphere
absorbs all of it. This in turn causes an almost instantaneous heating of this air to
hotter than the surface of the sun. The sudden expansion of this extremely hot gas
is what causes the fire and blast we witness when a nuclear device is detonated.

But in the vacuum of space, the blast effects from a nuclear explosion would be much
weaker. There would be no gasses to absorb the radiation and expand rapidly,
except for those from the vaporized bomb. Some blast and heat energy would be
absorbed by the comet, but the bulk of it would shoot off into space, maybe frying
the electronics of dozens of earth satellites. So, for a nuclear explosive to be
effective, it would have to be buried deep in the interior of the comet. Again, the
logistics of drilling a deep shaft in a comet millions of miles from the earth would
be a big problem. Also, the calculations below suggest that a single warhead,
which only contains a few Kg of nuclear explosive, does not produce sufficient
explosive energy to have a significant effect on a large comet.

A Possible Solution to the Problem

There is another possible way to use nuclear power to divert or even destroy
a comet. Remember the Chernobyl disaster in Russia, when a nuclear power plant
melted down. The movie, “The China Syndrome” introduced the idea of what
would happen if a meltdown could not be stopped. According to this theory, the
molten core would eventually melt it’s way through the earth’s crust and sink to the
center of the earth, spewing out superheated gasses as it descended.

What if we could put a very large mass (much more than found in a bomb) of
uranium 235 or plutonium on the surface of a comet? Then deliberately let it go
critical and melt down. If the meltdown was initiated slowly, the heat generated by
the core would vaporize the ice under it, and one would hope that it would sink into
the comet far enough so that escaping gas from a complete meltdown would not overcome the
weak gravity of the comet head and blow it into space.

When the nuclear material would have sunk sufficiently deep, a complete meltdown
would be initiated. Gravity would draw the hot molten core toward the center of the
comet. And at the same time, hot gasses would be blasted out the hole into space.
You would, in effect, have a giant nuclear rocket capable of firing for a very long
time. The energy output of this super rocket would be much greater than that of a nuclear
explosive because of the enormous mass of nuclear fuel and an unlimited supply of
propellent (steam). If a large enough quantity of nuclear material was used, it
might be capable of destroying the entire comet from the inside out.

There would, of course, be some danger in this type of operation. We would
have to be absolutely certain that the comet would be diverted far from the Earth’s
path. Otherwise, we could be hit by highly radioactive debris. It would be
necessary to do a great deal of experimentation with “safe” comets before such a
system could be used with any confidence. Even so, a danger of radiation would
probably be preferable to certain annihilation from a big comet.

An Additional Possibility

This type of process could be used for
transporting payloads weighing thousands of tons inexpensively through space.
Just intercept a comet or ice asteroid, load it with ores mined from metallic
asteroids, induce a controlled meltdown, and send it on it’s way. Of course, you
would have to find a way to steer it.

Calculations

Calculating the Mass to be Moved

First we will calculate the mass of a comet with a volume of one cubic kilometer.
We definitely would not want something that big to hit the earth. Scientists believe
that a comet is composed mostly of ice, and we know that
one cubic centimeter of ice weighs aproximately one gram.
So, 1 Cubic Km = 10 E15 Cubic cm, which is equivalent to 10 E12 Kg of ice.

In conclusion, our comet weighs 10 E12 Kg, or 1000 billion Kg

(Note that '10 E12' stands for ten to the twelfth power, or ten multiplied by itself
twelve times)

Calculating the Energy Produced

The nuclear reaction associated with fission of Uranium 235 is:

U (235) + n --> Cs (140) + Rb (37) + 3 n + 200 Mev

One Uranium 235 atom is hit by a neutron. It disintigrates, producing Cesium 140,
Rubidium 37, three neutrons and 200 million electron volts. The neutrons produced
hit other Uranium 235 atoms, making them undergo the same reaction. The chain reaction
continues until the remaining uranium atoms are spread so far apart that neutrons can
no longer hit them.

First, we will calculate the amount of energy produced by one Kg of Uranium 235.

Let's estimate how much useful energy we could generate if we planted one metric ton of
Uranium 235 on a comet. Also let's assume that only 10 per cent of the energy produced is
actually converted into motion. Therefore, the energy of motion generated by one ton of Uranium 235 will be:
(1.928 x 10 E10) x 1000 x .10 = 1.928 x 10 E12, or close to 2 x 10 E12 joules.

Calculating the Change in Velocity of the Comet

Now that we know the weight of the ice and energy produced, we can
calculate how much the velocity of the comet can be changed. We will want to move it 90
degrees from it's original trajectory so that it will miss the earth by 100,000 km. We
will want to move this object this far because the trajectories of comets are rather
unpredictable. Therefore, we will want to have a large margin of error.

We will use the Kinetic Energy Equation, solving for Velocity:

velocity = SQRT( ( 2 x energy)/mass))

(SQRT means Square Root)

velocity = SQRT( ( 2 x 2 x 10 E12) / (10 E12)) = 2 meters per second.

Converting this velocity to km per day, we get a velocity of 173 km per day. But we want
to move the comet 100,000 km. So, the number of days needed before impact is:

100,000 km / 173 km per day = 578 days.

So, our calculations show that the comet's velocity must be changed about one and half years
before the comet is scheduled to impact. I am not able to calculate how long it would take
to change the velocity of the comet. That would depend on the speed of the nuclear reaction.
What if you use more Uranium 235? If you inspect the equation, you can see that to
double the velocity change, you will need four times as much Uranium 235.

Calculating the Change in Velocity of a Smaller Comet

Consider a smaller object, for instance one 100 meters across. Smaller, but still
big enough to take out a city. The mass will be one one thousandth as much as that of
the one cubic km object, one billion Kg. Using the above equation:

So, the number of days needed before impact = 100,000 km / 173 km per day = 25 days.
Since this object will be close enough to accurately calculate it's motion,
you would probably be able to get away with moving it only a fraction of that distance.

The above calculations provide only rough approximations. A number of factors are not
taken into account. As I noted above, I do not have the information to calculate how much
time is needed for ten percent of the nuclear energy to be converted into actual motion.
Also, a good deal of energy is needed to raise the temperature of
the ice to it's boiling temperature. That energy is lost. Secondly, the boiling off
of the ice will reduce the mass of the ice chunk, allowing a greater change in velocity
than the calculations indicate. And the arbitrary efficiency of ten per cent is, of
course, only a guess.